1. Field of the Invention
The present invention relates to an automatic sampler, for example, an automatic sampler for introducing each sample into a liquid chromatograph automatically.
2. Description of the Related Art
FIG. 3 shows flow paths as important parts of a related-art automatic sampler for liquid chromatograph.
In FIG. 3, an automatic sampler has a 6-port 2-position flow path-switching rotary valve 1 which has six ports arranged at regular intervals, and two path positions for connecting every adjacent two of the six ports to each other. When the flow path-switching valve 1 is rotated, one of the two path positions is moved to the other to switch flow paths.
Sample vessels 3 each containing a liquid sample to be analyzed are arranged on a rack 31. A sampling needle 5 sucks in and collects the liquid sample from selected one of the sample vessels 3. The sampling needle 5 is connected to a pump 2 through both a looped flexible conduit pipe (hereinafter referred to as “loop”) 6 and the flow path-switching valve 1, so that the pump 2 gives suction force to the sampling needle 5. After the sampling needle 5 driven by a not-shown automatic drive mechanism sucks in the liquid sample in the position (sampling position) represented by the broken line in FIG. 3, the sampling needle 5 moves to the position (injection position) represented by the solid line in FIG. 3. In the injection position, the sampling needle 5 is inserted into an injection port 4. After the insertion, the sampling needle 5 is kept liquid-tight in the injection port 4.
A mobile phase liquid for the liquid chromatograph passes through the flow path-switching valve 1 from a liquid feed pump 91 via a mobile phase liquid feed flow path 7 and further flows into a column 92 via a column upstream side flow path 8.
The automatic sampler further has a rinse mechanism 10 which includes another valve (low-pressure valve) 11, and a rinse port 12. The rinse mechanism 10 plays the important role of rinsing the sample liquid from the sampling needle 5, etc. prior to analysis of a next sample to prevent contamination caused by the previous sample. The detailed description of the rinse mechanism 10 will be omitted because it is not directly related to the description of the present invention.
Introduction of each sample by the automatic sampler is performed in the following sequence.
(1) In the condition that the path of the flow path-switching valve 1 represented by the solid line in FIG. 3 is validated while the sampling needle 5 is located in the sampling position, the sampling needle 5 is dipped into the sample vessel 3 and the pump 2 is actuated so that a predetermined amount of the liquid sample is sucked in and collected. The collected liquid sample is mainly retained in the loop 6.
(2) The sampling needle 5 is moved to the injection position and inserted into the injection port 4.
(3) When the flow path-switching valve 1 is rotated by 60 degrees so that the path represented by the broken line in FIG. 3 is validated, the mobile phase liquid fed by the liquid feed pump 91 flows into the column 92 via the mobile phase liquid feed flow path 7, the loop 6, the sampling needle 5, the injection port 4 having the sampling needle 5 inserted therein, a pipe 41 and the column upstream side flow path 8 successively. As a result, the liquid sample mainly retained in the loop 6 is carried to the column 92 and analyzed.
(4) The flow path-switching valve 1 is rotated back and rinsing (which will be not described in detail) is performed by the rinse mechanism 10 to prepare for collection of the next sample. Then, the automatic sampler stands by for collection of the next sample.
The automatic sampler shown in FIG. 3 is called “direct injection type automatic sampler” because the sample sucked in and collected by the sampling needle 5 can be directly introduced into the column 92. In the related-art direct injection type automatic sampler, the injection port 4 and the flow path-switching valve 1 are disposed separately and connected to each other by the pipe 41. As described above, because the liquid sample must pass through the pipe 41, the inner volume of the pipe 41 forms dead volume. Hence, there is a drawback that the sample peak is spread. In addition, if there are piping joints in the path through which the sample passes, there is generally possibility that the sample may remain in fine gaps at the piping joints so that cross contamination occurs easily. Therefore, the number of piping joints needs to be reduced as extremely as possible.